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Research Article Adv. Mater. Lett. 2015, 6(10), 920-923 Advanced Materials Letters
Photovoltaic devices based on composites of conjugated polymer/fullerene have emerged as promising candidates
for solar cell applications [1]. The theoretical analysis of J-V characteristics is an important practice for understanding the charge conduction mechanisms and thus facilitating better device design for enhanced efficiency and stability. Various theoretical models have been suggested to explain
the behavior of OSCs [2-6]. In the present work we have attempted to explain the conduction phenomenon of P3HT:PC61BM, P3HT:PC71BM and PCDTBT:PC71BM
based solar cells with a single theoretical model [6].
Experimental
The solar cells were fabricated with vertical layer configuration as ITO/PEDOT:PSS/Active Layer/Aluminum. Indium tin oxide (ITO) (10 Ω/sq) coated glass substrates were cleaned with soap solution in ultrasonic bath and then rinsed in deionized (DI) water for 15 minutes each. The substrates were then boiled in acetone (10 minutes), isopropanol (10 minutes) and trichloroethylene (10 minutes), followed by an oxygen plasma treatment (5 minutes) prior to film deposition. A ~50 nm thick film of poly(3,4-ethylenedioxythiophene)-poly(styrene sulfonate) (PEDOT:PSS) was spin coated at 2000 rpm for one minute and then annealed in vacuum at a temperature of 120
oC for 20 minutes. For all the active
materials, o-dichlorobenzene was used as solvent. The
active layer blend ratios and concentrations have been
chosen from previously optimized conditions [7, 8]. The active layer of P3HT:PC61BM (20 mg/ml and 1:1 ratio) and P3HT:PC71BM (20 mg/ml and 1:1 ratio) were spin coated at 600 rpm for one minute to get a film thickness of ~150 nm (dried in vacuum oven at 100 °C for 30 min), while the active layer of PCDTBT:PC71BM (12 mg/ml and 1:4 ratio) was spin coated at 1000 rpm for one minute to maintain a film thickness of ~100 nm (dried in vacuum for 24 hours). The aluminum electrodes (~100 nm) were finally thermally evaporated in a vacuum of 10
-6 torr. Solar simulator,
SS50AAA, with 100 mWcm-2
intensity and Keithley-2400 source meter were used for taking J-V measurements. The measurements were taken immediately after aluminum contact deposition (referred as freshly prepared samples) at room temperature. All the chemicals are procured from Sigma Aldrich chemicals.
Theoretical Model
The continuity equation for electron current in dark is given by,
where, µ is the charge carrier mobility, n(x) is electron density, F is the electric field and Dn is the diffusion coefficient. This equation has been further solved by assuming a constant electric field both in the dark and
under illumination, and following equation has been
obtained for OSC in the dark [6],
(1)
where, JD is the dark current density, k is Boltzmann constant, T is temperature in Kelvin, n is the diode ideality factor, Vbi is the built-in voltage, V is the applied voltage, d is sample thickness, NC is the effective density of states, φ
are the electron injection barriers at the electrodes and Dn is the diffusion coefficient. Also where, μ is the charge carrier mobility.
Under illumination the J-V characteristics for OSCs is
given by [6],
(2)
where, JL(V) is the photo-generated current density and has been calculated under ideal conditions by,
τ is the lifetime of the charge carrier, JSC is the short
circuit current density.
Results and discussion
Fig. 1 shows the experimental current density-voltage (J-V) characteristics of P3HT:PC61BM OSC in the dark and under illumination.
(b)
(a)
Fig. 1. Experimental and fitted J-V characteristics of freshly prepared P3HT:PC61BM OSC (a) dark characteristics (b) illuminated characteristics.
The open circuit voltage values (Voc) and short circuit current density (Jsc) was found to be 0.38 V and 5.03 mA/cm
2 respectively. It was observed from the figure that
theoretical curves agree well with the experimental data. The realistic parameters used for the theoretical
calculations using equation 1 and equation 2 were [6, 9] T=295K, Nc = 10
19cm
-3, n=1.8, Vbi= 0.47V, d= 150 nm, τ =
2.5×10-5
s, μ = 9.9×10-5
cm2v
-1s
-1.
The J-V characteristics of freshly prepared
P3HT:PC71BM OSC was shown in Fig. 2. The values of Voc and Jsc for P3HT:PC71BM OSCs were found to be 0.44 V and 3.25 mA/cm
2 respectively. It was observed that the
experimental data is well explained by drift-diffusion model. The realistic parameters used for the fitting of
P3HT:PC71BM OSC were [6, 9] T=295K, Nc= 1019
cm-3
, n=1.7, Vbi= 0.45V, d= 100 nm, τ= 2.0×10
-5s, μ= 1.0×10
-4
cm2v
-1s
-1.
(a)
(b)
Fig. 2. Experimental and fitted J-V characteristics of freshly prepared P3HT:PC71BM OSC (a) dark characteristics (b) illuminated characteristics.
Fig. 3 shows the dark and illuminated J-V characteristics of PCDTBT:PC71BM OSC. It was observed that Voc has improved to 0.67 V and Jsc to 9.21 mA/cm
2.
Experimental J-V characteristics were shown to be in good agreement with the theoretical curves. The realistic
parameters taken for theoretical calculation were [10-11] T=295 K, Nc = 10
20 cm
-3, n=1.8, Vbi= 0.66V, d= 100 nm, τ
= 9.0×10-5
s, μ = 1.0×10-4
cm2v
-1s
-1. Higher value of Voc and
Jsc of PCDTBT:PC71BM OSC can be attributed to the larger difference between HOMO of acceptor and LUMO
of donor in these solar cells as compared to previous solar cells, which further influences charge dissociation and
charge transport [12].
(a)
(b)
Fig. 3. Experimental and fitted J-V characteristics of freshly prepared PCDTBT:PC71BM OSC (a) dark characteristics (b) illuminated characteristics.
Fig. 4 shows the energy band diagram of the three donor acceptor combinations used for the active layer of the devices. The theoretical value of Voc in conjugated polymer-fullerene solar cell is given by the following
empirical formula [13],
where, E is the energy level, e is the elementary charge, and 0.3 eV is an empirical value for efficient charge separation.
Equation 3 implies that greater the difference between HOMO of the donor and LUMO of the acceptor, higher
will be the value of Voc. It can be observed from Fig. 4 that the PCDTBT:PC71BM combination has highest energy difference which can be correlated with the highest Voc
obtained experimentally for this system. Further, the short circuit current (Isc) in OSC depends upon the dissociation efficiency of exciton, which in turn is determined by the offset between the LUMO of the donor and the LUMO of
the acceptor [14]. Generally, the minimum difference required between LUMO of donor and acceptor is 0.3 eV,
and any energy difference greater than this will simply
result in wasted energy [15]. Therefore, it can be observed
from Fig. 4 that PCDTBT:PC71BM has the optimum energy offset values, thereby giving highest values of Voc and Jsc as compared to the other two combinations. In conclusion, it is very interesting to observe that J-V characteristics of all freshly prepared solar cells can be exclusively explained by a single drift-diffusion theory.
Fig. 4. Energy band diagram of P3HT:PC61BM, P3HT:PC71BM and PCDTBT:PC71BM active layers.
Conclusion
Organic solar cells with different active layer materials were fabricated in the ambient conditions. Single theoretical model has been used to explain the J-V characteristics of various OSC. It was observed that the experimental curves were in good agreement with the theoretical generated curves using realistic parameters. Higher Voc and Jsc in PCDTBT:PC71BM OSC was due to the optimum difference between HOMO and LUMO of donor and acceptor material in these solar cells.
Acknowledgements The authors are grateful to Delhi University Research and Development Scheme 2014-2015 for providing financial support (RC/2014/6820). The corresponding author is thankful to Prof. Madan Mohan Chaturvedi, Director CIC, University of Delhi. The authors are further thankful to Dr. Rajendra Prasad, Principal, Ramjas College, University of Delhi for his kind support and encouragement. One of the authors (Vishal Sharma) is also thankful to University Grants Commission (UGC), Govt. of India and
Dept. of Higher Education, Govt. of Jammu & Kashmir for the encouragement and providing him study leave for pursuing research in organic solar cells.
Reference
1. Silva, W. J. D.; Kim, H. P.; Yusoff, M. A. R.; Jang, J; Nanoscale,